Tracking and receptor identification of the herbivore elicitor volicitin in plants

Volicitin (17-hydroxylinolenoyl-L-glutamine) present in the regurgitant of beet armyworm {Spodoptera exigua) activates the emissions of volatile organic compounds (VOCs) when in contact with damaged com {Zea mays) leaves. VOC emission in turn serves as a signaling defense for the plant by attracting female parasitic wasps that prey on herbivore larvae. Chemical tracking from the herbivore larva to the damaged leaf and interaction of volicitin within the plant has yet to be reported. First, we present biochemical data that beet armyworm regurgitant serves as a vector for the introduction of volicitin to the site of leaf damage under natural feeding conditions. Com seedlings were CO2 labeled in situ, and beet armyworm larvae were allowed to feed on the labeled leaves. Herbivore oral secretions collected from these larvae contained approximately 120 pmol volicitin (0.05 nCi/ pmol) per larvae. When radiochemically labeled larvae were placed on unlabeled leaves, the amount of volicitin introduced to the damaged site was approximately 5.0 nCi (calc. 100 pmol/larvae). The mobility of volicitin in leaves was examined by allowing radiolabeled beet armyworm to feed on unlabeled plants. In such tracking experiments, radioactivity was not detected in the upper leaves; however, the exogenous application of 5 nCi of [ ''C(U)]-sucrose to the lower leaf did result in subsequent radioactivity detected in the upper portion of the plant suggesting that, volicttin and other regurgitant components were not transported from the site of damage.